In a typical model of a crane, there may be three motors which need to be controlled independently; the motor that rotates the jib, another that raises or lowers the jib, and another that raises or lowers the hook. Each motor can be controlled by a separate switch that allows three positions forwards, backwards and off. This type of switch is called a Double Pole Double Throw (D.P.D.T.) switch with a centre-off position.
A switch like this will have six connections to it -- three for each pole of the switch and can be brought as a toggle type of switch or a slide type. The ideal switch for this application would have a momentary action (it would automatically switch back to the centre-off position after the operator has released the switch), but these are hard to find.
The circuit in figure 1 shows how to wire up one motor to a D.P.D.T. switch.
How it works
The switch in figure 1 is a Double Pole slide switch in which
connections A, B and C form one pole and connections D, E and F
form the other. Connections B and E are the common
connections for their respective poles.
If the positive power supply enters at B, and the switch is set
to the topmost position, the left-hand connection to the motor
becomes positive, and the motor will run in one direction.
If the switch is then set to the lowest position, the supply is
crossed over and the right-hand connection to the motor becomes
positive, and the motor runs in the other direction.
In the centre-off position, no power is connected to the motor.
See also: Reversing
using two relays
The relay (shown in figure 2) is
another type of switch in which the lever is operated by an
electromagnet. This is useful because a low voltage circuit can
energise the coil, which then throws the switch to turn on a
high-voltage (even mains) circuit driving high-voltage motors or
lamps.
Most relays need 9V or more to energise their coils, and they all
have maximum current ratings for their switch contacts which
should not be exceeded.
Advantages
of relays
Disadvantages
of relays
There are several connections to the relay, as shown in figure 3. The common connection is the one that will be connected either to NCor NO. NO means Normally Open, i.e., it is connected to common when the relay coil is energised. NC means Normally Closed, which means it is not connected to common when the relay coil is energised. Two other connections (usually separated from the rest) are to energise the coil (i.e., turn the relay on).
The latching relay circuit performs almost the same job as the 'bi-stable' micro-electronic circuit we will meet in a later issue. See figure 4. If the common of one of the relay poles is connected to +Vs and the NO is connected to the coil, when the relay is switched on a current will flow through the switch to the coil, keeping it energised even when the push switch is released. The only way to turn the relay off is to remove the power supply.
Uses in models
What's the point of this?
The NO contact of the relay could be connected to a motor. The
motor is able to turn indefinitely until some event (maybe
triggered by the motor) momentarily interrupts the power supply,
thereby turning off the coil ready for the push button to be
pushed again.
This system could be used in a model which needs a Push to
Operate button. A motor and gearing system in the model can
be used to depress another switch to cut the power to the relay
after the model has been running for a certain amount of time, or
until a certain event has occurred.
Model Study: The Chocolate Vending Machine
My model of a vending machine uses two of the latching relay circuits described above set up so that one chains to the other. The operation is as follows :
Other notes about the model
The switch S10 is a double-pole switch and the two poles are shown ganged with a dotted line. This switch simply bypasses the relays to switch on the motors indefinitely so that the machine can be emptied.
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Various photos of the Vending Machine (To view a photo full size, click on it) |
'All About...' is a column featuring different electronic components, since you will need to know how to use and identify components if you build some of the circuits described in future issues.
In this issue are some facts about resistors:
What they do
A resistor placed in a circuit will resist the passage of electrical current through the circuit and will therefore alter the voltages in the circuit according to Ohms Law...
V = I x R
...which relates voltage (V in Volts) to current (I in Amps) and resistance (R in Ohms). The quantity Ohms has its own symbol, the Greek letter Omega (W).
How to identify them
The circuit symbol for the resistor is shown below:
The value of a resistor (in Ohms) can be found by looking at the three coloured bands printed on it. A fourth band will also be found further apart from the others this is the tolerance and need not be considered.
All of the colour bands use the system shown below to obtain a number. The first band gives the first digit of the value, the second band gives the second digit of the value, and the third band gives the number of zeros which must be put after the first two digits. Thus, if the first band is BROWN, the second BLACK and the third ORANGE, the value of the resistor is :
1 |
0 |
000 |
= |
10,000W |
(Brown) |
(Black) |
(Orange) |
or 10KW |
This table shows the relation between the colours used on resistors and the value a colour represents Colour Value Black 0 Brown 1 Red 2 Orange 3 Yellow 4 Green 5 Blue 6 Violet 7 Gray 8 White 9
See
also: Identifying five-band resistors. Five-band
resistors
Need some help?!? Why not try the Resistor Colour
Code Wizard?
Only works on Microsoft Internet Explorer 4.0 or above.
Learning how to solder takes a lot of practice try making joints onto a stripboard track, and joining two pieces of wire using an old piece of stripboard and small lengths of wire.
Here are some guidelines about soldering:
Soldering tools
Soldering technique
When
you first use your soldering iron (or fit a new tip to it),
tin the tip with solder as the iron heats up. When
soldering two wires together or attaching a wire to a component,
a process called tinning is used, as shown in figure
6. Place the tip of the soldering iron on the wire and rest the
end of the solder on the wire (not the iron) until it melts onto
the wire. Remove the solder and the iron before too much solder
melts onto the wire. Do the same for the other wire or the
component lead. Touch the two wires together and run the iron
quickly over them. The solder on the wires should flow cleanly
and join the wires together.
If the joint breaks when you try to pull the wires apart, either
the wires were not tinned enough, or you moved the joint before
the solder dried.
When
soldering a component lead to a piece of stripboard or to a
Printed Circuit Board (PCB), push the lead through the stripboard
hole from the component side and allow about 5mm of wire to
protrude (see figure 7). Place the tip of the soldering iron at
an angle onto both the stripboard track and the component lead
and allow both to heat up. Feed the solder in around the track
and you should find that it runs all the
way around the hole and the component lead. If this does not
happen, the track or the lead are not hot enough. Allow only
enough solder to complete the joint too little and there
will be a visible weak spot, too much and there will be a solder
blob and you risk the solder splashing into other
tracks.
When the iron is removed the solder should look shiny and should
be in a pyramid shape.
The joint should not be able to move. Trim the component lead
down to the top of the solder.
The
following lists the electrical parts that are discussed in the
articles. Prices and order codes given are taken from the current
Maplin catalogue, which is the probably best source of electronic
components for the hobbyist in the UK.
If you have access to a company account with Rapid Electronics or RS Electronics you may find these companies are cheaper.
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Resistors can also be brought individually for around 6p each. |
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Maplin charge £2.50 for delivery
on orders under £30.00 inc. VAT.
Prices are taken from the September 2000 - August 2001 Maplin
catalogue, and include VAT at 17.5%
Contact their order line on 0870 264 6000
or visit one of their shops.
Their customer service line is 0870 264 6002 and
they have a website at www.maplin.co.uk where on-line ordering is
available.
www.eleinmec.freeserve.co.uk |
Electronics in Meccano September 1998 -- Issue 1 Edited by
Tim Surtell |
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